Some Viewpoints on the Phylogeny of Rust Fungi. III. Origin of Grass Rusts

Abstract

Serious economic losses caused by cereal rusts to our grain crops and field grasses are well known facts of much concern to plant growers. For example, it is estimated that six species of the genus Puccinia, which attack our main cereals, are causing more losses every year than all other rusts combined (Arthur, 1929, p. 326). This is due to the fundamental value of these crops, which form the main basis of the food supply for both man and domestic animals, and also to the wide-spread and aggressive nature of the rusts involved. Therefore, it is no wonder that the cereal rusts have been the object of most intensive study for centuries. In spite of the importance and the increasing scientific interest of the grass rusts, little is known about the phylogeny and evolution of this group of fungi. In most existent monographs and manuals, referred to at the end of this paper, little information is given about the origin and genetic relationships of grass rusts. Up to the present time, no generally accepted phylogenetic system has been proposed either for taxonomic classification or for practical identification of the enormous number of rusts on gramineous plants. Yet our most effective control measure, the breeding of rust-resistant varieties, depends ultimately upon our knowledge of the genetic constitution and phylogenetic relationships of the rusts involved. Through a series of genetic studies on the flax rust, Flor (1955, 1956) has recently shown that the host and parasite possess complementary genetic systems, which are based upon a series of multiple alleles at several gene loci. Any gene for resistance in the host plant acts only when there is a cor-